If you look on Google Earth you will see the Shackleton is well beyond the main gate and I doubt it would be visible from the road.
There were certainly Americans at St Mawgan, there was a US Naval Aviation Weapons Facility there, the guarding of which was supplemented by RAF personnel in the event of fog, which was quite common. Later the US took entire responsibility for guarding the facility, hence the Marines.
The weapons facility had a sister establishment at RAF Machrihanish. A spot of Googling should reveal more.